Recently, reduction of emissions of carbon-dioxide, which is considered as contribution to global warming prevention, has been required and thus use of a blended fuel of liquid hydrocarbon such as gasoline and ethanol as motor fuel has been investigated. As the ethanol, a bioethanol obtained by fermentation of a plant substance such as sugar cane and corn can be used. Since the plant substance used as a raw material itself has already absorbed carbon dioxide by photosynthesis, if ethanol obtained from such a plant substance is burned, the discharge amount of carbon dioxide is equal to the amount of carbon dioxide absorbed by the plant itself. In short, carbon dioxide emission on the whole theoretically becomes zero. More specifically, a so-called carbon neutral effect can be obtained. Therefore, if bioethanol is used in place of liquid hydrocarbon such as gasoline, carbon-dioxide emission can be reduced by the amount of bioethanol.
However, the sugar cane, corn and the like should be used fundamentally as food. If they are used as a raw material for bioethanol in a large amount, the amount of them supplied as food is reduced. This is a problem.
Therefore, a technique for producing ethanol using nonedible lignocellulosic biomass in place of a plant substance such as sugar cane and corn has been investigated. The lignocellulosic biomass contains cellulose. If the cellulose is decomposed by enzymatic saccharification into glucose and the obtained glucose is fermented, bioethanol can be obtained. Examples of the lignocellulosic biomass include wood, rice straw, wheat straw, bagasse, bamboo, pulp and waste materials (e.g., used paper) produced from these.
However, lignocellulose contains hemicellulose and lignin other than cellulose as major components. Usually cellulose and hemicellulose are tightly bound to lignin. It is difficult to apply an enzymatic saccharification reaction directly to the cellulose. Accordingly, when the cellulose is subjected to an enzymatic saccharification reaction, lignin is desirably removed in advance.
For removing lignin from lignocellulose, a saccharification pre-treatment device is known, in which lignocellulosic biomass is mixed with liquid ammonia and then the pressure is rapidly reduced to remove lignin from the lignocellulosic biomass (see, for example, Patent Literature 1).
In the conventional saccharification pre-treatment device, first lignocellulosic biomass is mixed with liquid ammonia by mixing means and the resultant biomass-ammonia mixture is heated by heating unit. Next, the biomass-ammonia mixture heated is pressurized and compressed by pressurization means so as for ammonia not to evaporate and discharged by discharge means.
If treated in this manner, the biomass-ammonia mixture is rapidly reduced in pressure with the progress of discharge and liquid ammonia is evaporated; at the same time, explosively expanded. As a result, the biomass is rapidly expanded to remove lignin bound to cellulose and hemicellulose of the biomass.
However, in the conventional saccharification pre-treatment device, since the biomass-ammonia mixture must be treated at a high temperature and a high pressure, it is difficult to treat the mixture in a continuous manner. This is a disadvantage. In addition, in the conventional saccharification pre-treatment device, in order to recover ammonia gas separated from the biomass-ammonia mixture and reuse it as liquid ammonia, the ammonia gas must be pressurized to about 2 MPa. Cost inevitably increases. This is another disadvantage.
To overcome the disadvantages, it is considered to use ammonia water in place of liquid ammonia. When ammonia water is used, lignocellulosic biomass is mixed with ammonia water to provide a biomass-ammonia mixture. Then, if the biomass-ammonia mixture is heated and boiled, the biomass expands by the expansion effect of the ammonia water by boiling; at the same time, the mixture is treated with alkali of the ammonia water to remove lignin.
Accordingly, when ammonia water is used, lignin can be removed by a boiling treatment without applying pressurization/compression. Therefore, the biomass can be easily treated in a continuous manner and cellulose of the biomass can be subjected to an enzymatic saccharification reaction without being inhibited by lignin.
Furthermore, when ammonia water is used, ammonia gas evaporated from the biomass-ammonia mixture boiled can be dissolved in water and recovered as ammonia water, and put in recycle use.
However, when ammonia gas is dissolved in water, heat of dissolution generates. If the temperature of ammonia water is increased by the heat of dissolution, the solubility of ammonia reduces. This is a problem and an improvement thereof is desired.
Citation List
Patent Literature
Patent Literature 1: Japanese Patent Laid-Open No. 2005-232453